The present invention relates to processing for the reproduction of a color image and, more particularly, to digital color image reproduction processing for recording halftone which separates an original image into different color components to produce image data, or density data, of the individual color components, converts the image data into record color component density data, specifies on a color component basis a particular halftone representation pattern based on the record color component density data, and records that pattern.
In one of halftone image recording systems known in the art, halftone is recorded on the basis of digital image data, or density data, having tones which range from 0 (zero) to M.N. Specifically, digital image data are individually compared with M.times.N threshold values which are distributed in an M.times.N matrix either regularly or randomly. When any of the digital image data is greater than its associated threshold value, a record data bit is assigned to the position of that threshold value in the matrix; when the former is smaller than the latter, a non-record data bit is assigned. More than one dot is assigned to each bit of the resultant record and non-record bit matrix which corresponds to the threshold matrix. In another prior art recording system of the kind described, a threshold matrix and image data which are representative of 1 to M.N are compared to produce beforehand M.N record and non-record data bit matrices each corresponding to a respective one of those conditions wherein the image data is 1 to M.N, the matrices being stored in a memory. In the event of image reading and recording, one of such matrices is specified by image data read, so that data is recorded based on that specified bit matrix. Monocolor recording requires only one threshold matrix or M.N record and non-record data matrix. In the case of color recording, e.g., color recording using yellow (Y), magenta (M) and cyan (C), Y image record data, M image record data and C image record data are produced by color separation and signal processing, the above-stated halftone recording being effected with each of the three-color image record data.
However, when halftone recording in a plurality of colors is implemented with the same threshold matrix or the same set of record and non-record data bit matrices, moire and others appear in a reproduced color image to degrade image quality and, moreover, all of the colors are superposed at the same point to lower the distinctness of the colors.
One approach known in the art to eliminate moire and others consists in rotating the threshold matrix or the record and non-record data bit matrices by each predetermined angle to provide modified matrices for each of the colors, the modified matrices being different in screen angle color by color.
Nevertheless, various problems have heretofore been left unsolved. Specifically, the distinctness of colors cannot be improved to a satisfactory degree because even in a low density recording area the different colors are recorded in a superposed relation at the same point. The number of tones available is limited so that sufficient representation of halftone cannot be provided in a reproduced image. While the representation of halftone may be promoted by increasing the value of M.times.N, such undesirably increases the recording area to be assigned to each of image data (data representative of the desity of the entire predetermined small area of a reproduced image), resulting in the enlargement of a reproduced image relative to an original image. In order to avoid such enlargement, it is necessary to increase the small area which should be read as one image data, but such leads to rough image reading and, therefore, to unfaithful image reproduction. Consequently, even if a wide range of tones are set up, the quality of image reproduction cannot be improved unless the area of each record dot is reduced. For the above reasons, it has been impracticable to increase the size of each halftone representation pattern, whether it be a threshold matrix or a record and non-record data bit matrix, beyond a certain limit.
Further, it has been customary to rotate the same halftone representation pattern, i.e., an orthogonal threshold matrix or an orthogonal record and non-record data bit matrix by a predetermined angle. This limits the freedom available in setting the screen angle and, thereby, prevents halftone representation patterns capable of enhancing the distinctness of colors from being achieved. The freedom mentioned is further limited because, as previously stated, each halftone representation pattern is relatively small.